Various hot wire mass air flow meters are known. In general, they include a resistance wire stretched across the air flow so that the wire is cooled by the air flow. The cooling of the wire changes the electrical characteristics of the wire in a manner proportional to the mass of air flow past the wire. For example, if the wire has a temperature sensitive resistance and can be heated to a given temperature using a known amount of current, any additional amount of current needed to maintain the temperature can be attributed to the cooling effects of the air flowing past it. Alternatively, the change in resistance of the wire can be measured while maintaining a constant current flow. Typically, a separate measurement of air temperature is made so that any change in the electrical characteristics of the wire can be correlated with the mass of air flowing past the wire.
While the accuracy of such systems in relatively clean air has been acceptable, dirt particles in the air have posed problems. Dirt particles can cause abrasion and errosion of the wire element eventually destroying it. Dirt particles can also change the electrical characteristics of the wire. For example, a layer of dirt on the wire can insulate it from the cooling effects of the air thereby destroying accuracy. Various solutions to this problem have been tried. For example, applying an usually high current to heat the wire can be used to burn off the dirt. However, this adds to the complexity of the system and is only a periodic, not a continuous, solution. Further, the wire must be built to be able to withstand this additional current. In fact, sometimes relatively high voltages required in this system have caused the wire to burn up.
U.S. Pat. No. 4,215,565 issued to K. J. Zanker recognizes that dirt may adversely affect the measurement of a fluid carrying the dirt. Inserted into a fluid carrying conduit is a probe having an inlet opening which faces downstream in the conduit. The inlet opening communicates with an outlet opening in the end surface of the probe. The main flow, in passing through the conduit, causes a sampling flow of the fluid to be withdrawn from the conduit, and to be passed through the probe from the inlet opening to the outlet opening, and so back to the conduit. As a result, the amount of dirt which will be carried in the sampling flow is reduced from that in the main stream.
U.S. Pat. No. 3,352,154 issued to R. S. Djorup teaches a fluid flow sensor constructed of a refractory body having a surface to which adhere an electrically conductive ring and a pair of electrically conductive strips. The strips and the ring are made of a material exhibiting a change in electrical resistivity as a function of temperature. The problem of reducing the amount of dirt hitting the sensor is not solved. However, it is recognized that the sensor must withstand abrasion by dust in the atmosphere and erosion by weather. Accordingly, the heated metal elements are of sufficient thickness to remain intact over a lengthy period despite abrasion and erosion. Such an approach does not solve the problem of deposition of dirt on an electrically conductive surface which will affect the accuracy of the device.
There still remains the problem of reducing dirt deposition on a heated wire positioned across the air stream flow in a hot wire mass air flow meter. None of the uncovered prior art provides a solution to this problem. Such drawbacks have limited the use of the hot wire meter outside of the laboratory. However, it would be advantageous to develop a hot wire meter which could be sufficiently accurate in a relatively dirty environment so that it could be used as part of an engine control apparatus for an automobile.